The invention relates to a waveguide type duplex filter having at least first and second waveguides which each comprise filtering means tuned to a given frequency, wherein the duplex filter has two mechanical halves with a conducting surface, wherein faces directed toward each other are formed with channels which, when the halves have been assembled, constitute said waveguides.
Duplex filters of the type to which the invention relates are used e.g. in connection with radio link terminals, where they ensure that a radio link terminal can transmit on one frequency and receive on another at the same time. Within a given frequency band, one of the two terminals, which constitute a radio link, transmits on a high frequency and receives on a low one. Similarly, the other terminal of the radio link receives on the high frequency and transmits on the low one in order for the two terminals to be able to communicate.
The corresponding duplex filters must reflect the same condition and must therefore be present as pairs, where one duplexer has a filter on the transmit side for a high frequency and a filter on the receive side for a low frequency. The other duplexer has a receive filter for the high frequency and a transmit filter for the low one. The receive filter prevents the strong signal from the transmitter from overloading the receiver, and the transmit filter must primarily restrict transfer of noise to the receiver and distortion products to the antenna.
A duplex filter may e.g. be realized with two separate filters which are connected to the antenna through a so-called circulator. The individual subfilters or filter branches can be manufactured in different ways. In case of high frequencies, which means frequencies higher than 7 GHz in practice, they may be formed in waveguide structures by means of different techniques. A filter may e.g. be a waveguide in which a plurality of resonators in the form of small sections of the waveguide are directly coupled to each other by means of coupling elements in the form of inductive irises, which are soldered on the walls of the waveguide at a mutual distance of e.g. half a wavelength.
A waveguide filter may also be constructed as a so-called E-plane filter, where a pattern, which defines the characteristic of the filter concerned, is produced in a metal sheet arranged between e.g. two metallic halves. The waveguide itself is formed by means of channels in the two halves. The metal sheet thus defines some partition walls in the waveguide which, in the same manner as inductive irises, constitute coupling elements between a plurality of resonators in the form of small sections of the waveguide. Such an E-plane filter is known e.g. from U.S. Pat. No. 4,800,349.
Another possibility of constructing a waveguide filter is to use dielectric resonators which are mounted in the waveguide.
The national authorities"" allocation of frequency bands and specific frequencies for radio links normally means that the filters must be designed to cover just a portion of the allocated frequency band. In some of the above-mentioned filter types, the approach is that they can be trimmed with screws for a specific subband which contains the allocated frequency. The trimming is necessary partly to reduce the number of mechanical variants and partly to compensate for mechanical tolerances.
A duplex filter in waveguide technique may also be realized as an integrated unit consisting of a receive filter and a transmit filter which are coupled together via an adapted T-member to a common antenna port. The adaptation in the T-member ensures that the filters generate the desired frequency when they are coupled together.
Such an integrated duplex filter is known e.g. from EP 274 859. This document: describes a duplexer which consists of two mechanical halves of aluminium in which channels are formed. When the two halves are assembled, the channels, which are arranged opposite each other in the two halves, form waveguides which connect a receive port and a transmit port, respectively, with an antenna port. The filter effect is achieved here in that each of the two waveguides is provided with a plurality of stubs which constitute serial short-circuits at frequencies within the passage area of the filter, and which constitute finite reactances at other frequencies and thereby cause reflection. The number and dimensions of the stubs define the characteristic of the filter, such as centre frequency and bandwidth, and since the centre frequencies of the transmit filter and of the receive filter, respectively, are different, the stubs are thus also dimensioned differently in the two subfilters.
Although the trimming may be avoided by this duplexer, it is a serious drawback that since the stubs defining the characteristic of the individual filter are milled in the aluminium halves, these have to be manufactured individually for each frequency. The two duplex filters, which are to be used at their respective ends of a radio link, are different, too, since, as mentioned above, one must have a transmit filter with a high frequency and a receive filter with a low frequency, while the reverse applies to the filter at the opposite end. Thus, with this duplex filter type, a large number of different aluminium halves have to be manufactured in order to be able to build a system that utilizes the available channels within a given frequency band. Even the two duplex filters at their respective ends of a connection must consist of different parts, as mentioned.
Thus, an object of the invention is to provide a duplex filter of the type stated in the opening paragraph, where the necessary number of duplex filters for a system which utilizes the available channels within a given frequency band, may be manufactured by means of considerably fewer parts than in the prior art, and where at least the two duplex filters at their respective ends of a connection may be manufactured by means of identical parts.
This is achieved according to the invention in that said filtering means are produced by means of a sheet having a conductive surface which is arranged between the two halves, said conductive surface being provided with openings which define and delimit resonators in the two waveguides; that the sheet can at least be arranged in first and second positions; that, in the first position of the sheet, the resonators in the first waveguide are tuned to a first frequency, while the resonators in the second waveguide are tuned to a second frequency; and that, in the second position of the sheet, the resonators in the first waveguide are tuned to the second frequency, while the resonators in the second waveguide are tuned to the first frequency.
When such a sheet is used, the two duplex filters at their respective ends of a connection may be manufactured from just three parts, viz. the two mechanical halves and a sheet. The sheet is merely to be arranged in the first position in one filter and in the second position in the other filter. Moreover, the same two mechanical halves may be used for duplex filters for all pairs of frequencies in an entire frequency band, as only the sheets are frequency-specific. Thus, it is just necessary to stock the two halves as well as a plurality of sheets to be able to manufacture all the necessary duplex filters. And since the sheets just constitute a very small share of the total costs of a duplex filter, considerable savings can be achieved in this way.
It is expedient that, as stated in claim 2, the two mechanical halves consist of an electrically conductive material, since the halves may then be made exclusively of this material. As stated in claim 3, the said electrically conductive material may be aluminium in a particularly expedient embodiment, since this is inexpensive, easy to work and conducts well. In an alternative embodiment, as stated in claim 4, the electrically conductive material may be steel, which is likewise inexpensive, and which is moreover unique in having a low coefficient of temperature expansion.
As stated in claim 5, said sheet may expediently be a metal sheet, since this partly ensures a low sheet manufacturing price and partly means that said openings in the conducting surface may be produced by means of a range of various techniques, such as etching, laser cutting and the like. Alternatively, as stated in claim 6, said sheet may be a non-conducting sheet having an electrically conductive layer applied thereto. This means a more flexible sheet, and also provides the possibility that the openings may either extend through the entire sheet or just occur in the applied electrically conducting layer, as needed.
When, as stated in claim 7, said sheet has an area which corresponds to the area of the two halves where they are in contact with the sheet, a simpler assembly of the duplex filter is achieved, as the means holding the two mechanical halves together can simultaneously hold the sheet in place in its correct position relative to the halves. This obviates special control devices for ensuring the correct position of the sheet.
As stated in claim 8, the waveguides may expediently be provided with ports for the connection of external components, such as a receiver, a transmitter and an antenna, so that these may be connected directly to the duplex filter without further intermediate members. When additionally, as stated in claim 9, one or more of said waveguides have a bend so that the associated port is positioned solely in one of said halves, it is ensured that the flanges of the ports concerned are removed from the section that is present between the two halves. It is hereby considerably easier to make the flanges plane and tight, which is decisive for achieving sufficient isolation between transmitter and receiver.